Electron discharge device with temperature-stabilized cathode



May 8, 1951 J. KURSHAN ELECTRON DISCHARGE DEVICE WITH TEMPERATURE-STABILIZED CATHODE Filed April 19, 1949 SPEAKER 6Q 400/0 AMP! lF/ER F. AMPL lFlf/Z MIXER ALF.

NH-WIVA AMPLIFIER LOG/1i 030/114 70R INVENTOR JEROME KURSHAN Patented May 8, 1951 ELECTRON DISCHARGE DEVICE WITH TEMPERATURE-STABILIZED OATH- ODE Jerome Kurshan, Princeton, N. 3., assignor to Radio Corporation of America, a corporation of Delaware Application April 19, 1949, Serial No. 88,406

6 Claims. 1

This. invention relates to electron discharge devices and is particularly concerned with the partial or complete stabilization of the temperature of a bombarded cathode.

A conventional bombarded cathode structure comprises a primary cathode having a thermionic emitting surface for supplying electrons to be utilized in the electron discharge device, and a secondary or auxiliary cathode adapted to emit electrons for bombarding the primary cathode. The primary cathode is maintained at a positive DOtBntial with respect to the auxiliary cathode, as a consequence of which bombarding electrons trike the primary cathode and heat it to a temperature which is a function of the instantaneous value of said potential.

In certain applications, for example, in the case of aircraft communications equipment, electrode potentials are subjected to pronounced variations due to. variations. in the power suppl voltage, which, in some cases, may result in cathode temperature variations so extreme as to either render the device temporarily inoperative or shorten tube life, or both.

According to the present invention, the temperature of. a primary emitting surface is at least partially stabilized by supplying the bombarding voltage through an appropriate impedance in series with the power supply and the two cathodes, by employing as an auxiliary cathode a thermionic emitter. arranged for temperature limited operation, and by supplying energy to the auxiliary cathode at a rate which is a function of the voltage of the power supply.

As a result, changes in the power supply voltage, which would ordinarily result in changes in the velocity of the bombarding electrons and corresponding changes in the temperature of the emitting surface, are accompanied by compengating changes in the rate of electron emission by the auxiliary cathode. Changes in the rate of emission may be considered to be changes in the bombarding current; and since the external path of this current includes an appropriate impedance, changes in the rate of emission will result in compensating changes in the potential drop across the two cathodes.

When the rate at which energy is supplied to the auxiliary cathode varies in the same sense as the power supply voltage, the effect of changes in emission acts in opposition to the effect of changes in the power supply voltage, with the result that the temperature of the primary emitter surface is at least partly stabilized.

The external impedance may have a purely resistive component, or, under some circumstances, it may be desirable to employa circuit element whose dynamic resistance is greater than its static resistance.

According to one embodiment of the invention, the auxiliary cathode is maintained at operating temperature by radiation from the primary cathode, while, according to another embodiment, the auxiliary cathode is heated by an electric current which varies with the voltage of the sourse which supplies the bombarding voltage between the two cathodes.

It is therefore a general object of my invention to eliminate or to reduce changes in primary emitter temperature which would ordinarily result from change in the bombarding potential difference, by employing as an auxiliarycathode a temperature limited thermionic emitter whose energy, input varies in a suitable manner with the power supply voltage.

Another object of the invention is to provide an electron discharge device of the bombarded cathode type in which the auxiliary cathode is disposed to be maintained at a temperature appropriate to temperature limited operation by radiation from the primary cathode.

Another object of the invention i to stabilize the temperature of the bombarded cathode by supplying energy to an auxiliary cathode at an average rate appropriate to the maintenance of temperature limited operation of said auxiliary cathode, but subject to variations related to variations in the power supply voltage.

A still further object of the invention is to stabilize the temperature of a bombarded cathode by operatin the auxiliary cathode in the temperature limited portion of its characteristic, the said auxiliary cathode being heated by a heating current supplied by the same source as the bombarding voltage.

How these objects and others which will appear are attained will be seenmore fully from the detailed description which follows and from the the bombarding current to the primary cathode; and

Figure 3 is a schematic diagram illustrating a typical application of the invention to a known type of radio receiver.

In Figure 1, I have illustrated an envelope I containing an anode III, a primary cathode II adapted to emit electrons which may be collected by anode III, and an auxiliary cathode I2 which may be either a directly or indirectly heated thermionic cathode. It will be understood that one or more additional electrodes may be located between anode II) and cathode II, such additional electrodes being omitted from Figure 1 for the sake of clarity.

I have also indicated a source I3 of bombarding current and an auxiliary source I4 of current adapted to be connected across the terminals of auxiliary cathode I2 through a switch I5. Source I3 is series connected with a resistance It and the internal diode comprising cathode I2 and cathode I I, and provides for maintaining cathode II at a positive potential with respect to cathode I2.

Cathode I2 is adapted to be initially brought to whereupon the discharge device comprising primary cathode II and anode I operates in the usual manner.

In addition, cathodes II and I2 are so arranged that suflicient heat is radiated inwardly by cathode II to maintain cathode I2 at emitting temperature, and, accordingly, switch I may now be opened.

Assuming a stead value for the voltage of source I3, both cathodes will be maintained at emitting temperature, and the electron emission from cathode II will, of course, be constant.

Now consider the eifect of a change in the voltage of the source I3.

If the potential difference between cathode I2 and cathode II increases, due to an increase in the voltage of source I3, no increase in electron current from cathode I2 will result directly therefrom, because the cathode I2 is operated in temperature-limited condition. However, the increase in voltage will cause the velocity of electrons striking cathode II to increase, raising the temperature of cathode II which, at least temporarily, increases the electron emission from cathode II. At the same time, however, the increase in temperature of cathode I I is communicated in part by radiation to cathode I2, raising the temperature of the latter and resulting in an increased electron emission therefrom and, hence,

I6. If the diode potential drop could be made to vary exactly inversely with the diod current the power input to the cathode II would be constant, and the temperature of the cathode would be perfectly stabilized. While this ideal condition cannot be fully attained in th embodiment of my invention illustrated in Figure 1, it will be shown that, if the resistance it is large compared to the normal diode resistance, the effects of variations in the power source on the diode potential difference, and hence, on the power input to the primary cathode and on the cathode temperature, will be reduced to a minimum.

Upon decrease of cathode operating potential difierence, the velocity of electrons striking cathode II will decrease, the temperature 01" cathode II will decrease, the temperature of cathode I2 will also decrease, reducing the rate of emission of electrons by cathode I2 and increasing the voltage drop across the diode, as a result of which, again, fractional change in potential drop across the diode will be less than the fractional change in potential drop across the resistance I5.

It will be understood that since cathode I2 is heated by radiation from cathode II, its temperature will always be lower than that of cathode II, the difierence in temperature being great enough to permit the transfer of suihcient energy to overcome the work function of the material of which the emitting surface of cathode I2 is made.

While the two cathode structures ma be made of similar materials, it is preferable to form the emitting surface of the cathode I2 of a material having a higher work function than the material comprising the emitting surface of cathode II, in order that the dependence of the rate of electron emission by diode I2 upon temperature be as great as possible, since it is this factor which ultimately effects the stabilization of the temperature of the emitting surface of cathode II.

The principal requirement is that the operating temperature of cathode I2 be so related to its work function as to provide for temperaturelimited operation within the range of conditions to be met in operation. Since cathode II should be maintained at sufiiciently high temperature to give space charge limited operation, this means that if cathode I2 is operated at a temperature near the temperature of cathode II, the work function of the material of cathode I2 must be higher than that of the material of cathode II. Conversely, if the electron-emitting materials on the two cathodes are the same, the internal cathode should be arranged to operate at a considerably lower temperature than the external cathode. For example, the temperature of cathode I2 relative to that of cathode II may be readily controlled by control of the amount of heat conduction to the supports of cathode I2.

Where E0 is the instantaneous voltage of source I3, and R is the resistance of resistor I6, the power into the cathode II is From (2) it can be seen that the rate of change of power with current is negative if the D. C, resistance of the inner diode is less than the series resistance I6. Thus, the device will always be self-stabilizing if the internal resistance of the diode is small enough.

The magnitude of the stabilizing action to be expected in a typical example will be understood from the following. Assume that cathodes I I and I2 are to be operated at about the same temperature T, and let i be the temperature-limitedelectron current from cathode l2, and P the electron bombardment power to cathode I I. By the Stefan-Roltzmann equation P=K T (3) and by the Richardson-Bushman equation i=AT e- 4 It is the reciprocal of this quantity which we wish to make large.

Now,

dE =2E T+b(2RtE (7) but if cathode I I were heated by a zero tempera ture coefficient cathode heater in series with resistor IE,

We wish ('7) to be larger than (8), and, accordingly, we require 2Ri Eo. That is, the diode plate resistance must be less than the resistance of resistor I0. For economy, let th resistance of resistor I6 be only twice the diode resistance, or

and

Now, b=1l,600Ew (K), where Ew equals the work function in volts. In an oxide coated emitter,

we may take E =l volt 11 %=11.o at 1000 K. 12 and 11E, 11.6 2 T 2E T l+ 6 (13 Equation 13 indicates a reduction in the rate of temperature increase with increase in supply voltage of about 3 as compared with Equation 8.

It will be understood that dilierent degrees of stabilization will be achieved with differing values for resistor I6, and also for emitter materials having different work functions. For example, when employing tungsten for the emissive material 0f cathode I2,

and the reduction factor would be 1+22/6 or about 4%.

It should be understood that during operation of the tube it may not be necessary to disconnect power source I l, if the power derived from source I4 is small as compared with the bombardment power to cathode I I.

A typical example of a miniature receiving tube incorporating this embodiment of the invention may comprise a cathode I I having inside diameter have the reasonable diameter of .010 inch.

. current.

of 0.040 and a length of 25 mm., requiring a power input of 1.9 watts. The bombardment current corresponding to this amount of power must be less than the space charge limited current at the operating bombardment voltage, that is, the bombardment current must be less than the current which could be drawn by the bombardment voltage upon an increase in the temperature of the internal cathode. This condition may be met below 30 volts; for example, with a combardment current equal to 0.1 ampere/0111. cathode I2 may It will be understood that cathode If. may be made larger in diameter and operated at a lower temperature to give the same electron emission.

Turning now to the second embodiment of the invention, I have illustrated in Figure 2 a primary cathode Not, an anode ida adapted to colalso supplies bombarding current to the internal diode comprising the two cathodes. It will therefore be understood that the voltage applied to cathode IZa will vary directly with the voltage of the source 13a which supplies the bombarding I have also illustrated in Figure 2 a pair of additional grid electrodes ll and I8 interposed between cathode H and anode IBa, although it will be understood that the electron stream between cathode Ha and anode Illa may ,be controlled in other ways or that the primary electron stream may be entirely unintercepted.

It will be seen at the outset that qualitatively the embodiment of Figure 2 is capable of an operation similar to the embodiment of Figure 1, that is, the variations in diode resistance produced by changes in the energy input to the auxiliary cathode will oppose changes in bombardment voltage due to changes in the voltage of the bombarding current source. However, the result is produced in somewhat different manner in Figure 2. An increase in the supply voltage E0 would be expected to increase the potential difference between the two cathodes, and hence, the power to cathode Ho and the temperature thereof. However, the increase in E0 is also applied directly to auxiliary cathode I 2a in this ance I611 should be at least equal to the diode resistance, to produce any stabilizing effect. It will be shown that the resistance can be so chosen that the power input to the primary cathode will be constant, thus efiecting perfect stabilization of the cathode temperature.

If the filament of cathode lZa has a zero temperature coefficient of resistance, the power supplied to the filament I2a is E 2 PF: PTF

where m is the fraction of E0 applied to the filament, E is the power supply voltage, RF is the filament resistance at temperature TF, and p is a constant. With temperature-limited emission from the filament, the bombarding current is where A and b are the constants of Equation 4. From Equation 14,

T qE 16 Hence, ma be written i=BE e 7) The power to cathode I la is Pc=iE i R =BE e" C/Eomh BRe- Therefore, by difierentiating (18) and substituting back from (17) the rate of change of cathode power to change in supply voltage Eu, becomes dP 2iR iR -1/2 dEO l/2E 0%(1 Eu )+2r(1 E0 To find the optimum value of the resistance R for stabilizing the cathode temperature, let

where a; is the fraction of E0 appearing across R, and Equation 19 becomes dP fi=1/2Eg CZ(12$)+2Z(1$) (20) For perfect stabilization, dP/dEo must be zero, hence For a tungsten emitter,

the external resistance is only slightly greater than the diode resistance. A still further improvement may be achieved if the filament has a negative temperature coefiicient of resistance.

In either of the embodiments of Figure 1 and Figure 2, a substantial amount of energy is dissipated in the external resistance. When it is desirable to decrease the amount of energy lost in this manner, the external resistance may take the form of a circuit element Whose dynamic resistance is greater than its direct current resistance, in which case the same degree of stabilization can be achieved with a smaller loss of energy in the external resistance.

The invention is applicable, in general, to circuits employing electron discharge devices which must be operated from sources of electrical energy whose output voltages are subject to considerable variation. An example of such an application is illustrated in Figure 3, wherein I have indicated schematically an aircraft radio receiver of the superheterodyne type.

In Figure 3, I have illustrated by general designations the antenna, R. F. amplifier, mixer, I. F. amplifier, audio amplifier, and reproducer portions of the receiver, and have shown in some detail a local oscillator circuit employing an electron discharge device of the type illustrated in Figure 1.

If, as is customary, the cathode heating power is derived from the main battery of an aircraft electrical system, wide variations in voltage may be encountered. In the circuit of Figure 3, the aircraft battery is illustrated by the source 13. As a consequence of the operation discussed hereinabove in connection with Figure 1, the effect of variations in the voltage of battery [3 on the temperature of cathode Ila is greatly reduced.

Iclaim:

1. An electron discharge device of the bombarded-cathode type, comprising a main electronemitting element and an auxiliary electron-emitting element adapted to temperature-limited operation for supplying bombardment electrons for the main emitting element, in which the auxiliary element comprises an emissive surface having a higher work function than the emissive surface of the main element and in which the auxiliary element is disposed in a position relative to the main element to provide for maintaining the auxiliary element at a temperature approaching the temperature of the main element.

2. In combination, a first thermionic cathode, a second thermionic cathode adapted to temperature-limited operation arranged to supply bombardment electrons for maintaining said first cathode at emitting temperature and disposed to be heated by radiation from said first cathode to a temperature substantially below the temperature of maximum practical emission, and an electrical circuit including said two cathode and including a source of E. M. F. for maintaining a positive charge on said first cathode sufiiciently large to attract all electrons emitted by said second cathode, said circuit including an impedance element having a resistance which is large compared to the plate resistance of the diode comprising said two cathodes.

3. An electrical network comprising an electron discharge device of the type having a bombarded primary cathode and an auxiliary cathode, adapted to temperature limited operation, for supplying bombarding electrons, said network further comprising a single source of electrical energy in series with a resistance element and said cathodes for maintaining said primary cathode at a positive potential with respect to said auxiliary cathode and connected to said auxiliary cathode for delivering electrical energy to said auxiliary cathode at a rate proportional to the instantaneous value of the potential applied to said resistance element and said cathodes, the resistance of said element being at least equal to the plate resistance of the diode comprising said two cathodes.

4. An electrical network comprising an electron discharge device of the type having a primary cathode adapted to be maintained at a temperature appropriate to space charge limited operation by electron bombardment, and an auxiliary cathode, adapted to temperature-limited operation, for supplying bombarding electrons, in combination with a resistance element, a source of electrical energy in series with said resistance element and said cathodes for maintaining said primary cathode at a positive potential with respect to said auxiliary cathode, and means for delivering electrical energy to said auxiliary cathode from said source at an average rate appropriate to the maintenance of said auxiliary cathode in temperature-limited operation, the resistance of said element being substantially greater than the plate resistance of the diode comprising said two cathodes.

5. An electrical network comprising an elec- 6. An electrical circuit comprising an electron discharge device of the type having a primary cathode adapted to be maintained at emitting temperature by electron bombardment and an auxiliary cathode adapted to temperature-limited operation for supplying bombarding electrons, in combination with an impedance element having a resistance at least equal to the plate resistance 10 of the diode comprising the two cathodes, a first source of electrical energy connected in series with said impedance element and said diode for maintaining said primary cathode at a positive potential with respect to said auxiliary cathode, and a second source of electrical energy connected to said auxiliary cathode for producing temperaturelimited emission therefrom, whereby changes in the temperature of said primary cathode produced by variations in the voltage of said first source are reduced.

JEROME KURSHAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 20 1,210,678 Nicholson Jan. 2, 1917 1,585,766 Chubb May 25, 1926 1,699,146 Hull Jan. 15, 1929 2,239,416 Ehrenberg Apr. 22, 1941 2,250,511 Varian et'al July 29, 1941 25 2,284,405 McArthur May 26, 1942 2,348,814 Herriger May 16, 1944 2,419,536 Chevigny Apr. 29, 1947 FOREIGN PATENTS 30 Number Country Date 141,706 Great Britain Jan. 27, 1917 

